Ink-Paper Interactions in Printing: A Review - ACS Publications

20 Ink-Paper Interactions in Printing: A Review Downloaded by UNIV OF GUELPH LIBRARY on June 25, 2013 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch020

M. B. L Y N E and J. S. ASPLER Pulp and Paper Research Institute of Canada, Pointe Claire, Quebec, Canada

P h y s i c a l and surface chemical models f o r the a p p l i c a t i o n of ink and fountain s o l u t i o n to paper under p r i n t i n g c o n d i t i o n s are r e viewed. I n i t i a l t r a n s f e r of ink to paper i s p r i m a r i l y by h y d r a u l i c impression, and subsequently by wetting, adhesion, and f i l m splitting. The e f f e c t s of c a v i t a t i o n and f i l a m e n t a t i o n on the u n i f o r m i t y of ink t r a n s f e r and the i n f l u e n c e of ink rheology on t r a n s f e r and l i n t i n g and p i c k i n g are reviewed. Wetting delays f o r water and f o u n t a i n s o l u t i o n are discussed and the mechanism by which s i z i n g and s e l f - s i z i n g of paper increases the wetting delay i s d e s c r i b e d . The implications of f o u n t a i n s o l u t i o n wetting delays f o r m u l t i c o l o u r o f f s e t p r i n t i n g are described. P o s t - p r i n t i n g nip c a p i l l a r y s o r p t i o n of ink and ink v e h i c l e s i s discussed using Lucas-Washburn theory and the i n f l u e n c e of the rate of c a p i l l a r y s o r p t i o n on ink h o l d out, show through and set o f f are discussed. Finally, the long-term migration of oil v e h i c l e s over f i b r e surfaces by spreading with the attendant loss of paper opacity i s described.

0097-6156/82/0200-0385$09.95/0 © 1982 American Chemical Society In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Downloaded by UNIV OF GUELPH LIBRARY on June 25, 2013 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch020

The commercial p r i n t i n g industry embraces a wide v a r i e t y of processes f o r p u t t i n g ink on paper. This review w i l l be concerned with l e t t e r p r e s s and o f f s e t lithography i n t h e i r conventional forms. Ink formulations, p r i n t i n g forms, and p r i n t i n g pressures d i f f e r c o n s i d e r a b l y i n these processes, but the ink i s i n v a r i a b l y t r a n s f e r r e d from the p r i n t i n g medium to paper i n a p r i n t i n g nip. At f u l l commercial p r i n t i n g speeds paper passes through t h i s p r i n t i n g nip i n approximately one m i l l i s e c o n d . As depicted i n F i g u r e 1 f o r the l e t t e r p r e s s case there are s e v e r a l f a c t o r s i n volved i n the t r a n s f e r of ink to paper and subsequent migration of ink and ink v e h i c l e s i n the paper s t r u c t u r e . As shown i n F i g ure 2 an asymmetric pressure pulse i s created during the passage of the paper through the nip which both compresses the porous network s t r u c t u r e of the paper and h y d r a u l i c a l l y impresses the ink i n t o the compressed pores i n the surface of the paper. Penet r a t i o n rates of between 250 and 525 um/s f o r the simulated l e t t e r p r e s s p r i n t i n g of newsprint have been reported. This may be compared to 0.1 um/s f o r penetration 10 seconds a f t e r p r i n t ing I. Inks are t h i x o t r o p i c and pseudo-plastic so the h y d r a u l i c imp r e s s i o n of the ink a l s o depends on the shear h i s t o r y of the ink i n the i n k i n g system of the p r i n t i n g press and on the shear cond i t i o n s i n the p r i n t i n g n i p . Paper i s a v i s c o e l a s t i c m a t e r i a l and thus the shape and duration of the pressure pulse a f f e c t i t s compression i n the n i p . On the outgoing side of the nip subatmospheric pressures are created which cause c a v i t a t i o n and the onset of f i l a m e n t a t i o n i n the ink f i l m . The t r a n s f e r process i s complete with the f r a c t u r e of the lengthening ink f i l a m e n t s . Since the sub-atmospheric pressure pulse i s much smaller than the p o s i t i v e pressure pulse ink which has been forced i n t o the c a p i l l a r y s t r u c t u r e i n the paper on the ingoing side of the nip i s not withdrawn s i g n i f i c a n t l y on the outgoing s i d e . H y d r a u l i c impression of ink i n t o the surface c a p i l l a r i e s of paper does not r e q u i r e that the ink wet the surface of the c a p i l l a r i e s , but uniform ink f i l m s p l i t t i n g does r e q u i r e the ink to wet and adhere to the surface of the paper during the m i l l i s e c o n d of contact i n the p r i n t i n g n i p . In f a c t the u n i f o r m i t y of the p r i n t e d image depends on the ink contacting and wetting the s u r face of the paper i n the nip, on the r h e o l o g i c a l p r o p e r t i e s of the ink during l a t e r a l spreading and f i l m s p l i t t i n g processes, and on l o c a l v a r i a t i o n s i n the amount and depth of ink impressed i n t o the surface c a p i l l a r i e s i n the paper. A s l i g h t r e d i s t r i b u t i o n of the ink can occur a f t e r the paper leaves the p r i n t i n g nip due to a s p i r a t i o n as the network s t r u c -

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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0

Hydraulic impression • Wetting - Cavitation -Film splitting

"•y"

Relaxation

Compression ©

Network relaxation -Aspiration • Meniscus reversal

(D Capillary imbibition

Figure 1. A simplified model of ink impression and absorption.



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Nip center

Pressure Distribution Figure 2.

Pressure distribution and velocity profiles in the splitting of a film of Newtonian fluid.



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ture of the paper r e l a x e s . I n i t i a l c a p i l l a r y i m b i b i t i o n may a l s o cause ink to be drawn i n t o the paper, but the o i l v e h i c l e i s soon drawn i n t o p r o g e s s i v e l y f i n e r c a p i l l a r i e s s e t t i n g the ink by i n c r e a s i n g the pigment c o n c e n t r a t i o n - . When o i l v e h i c l e s move i n t o pores which would otherwise s c a t t e r l i g h t the o p a c i t y of the paper decreases. In t h i s way, i t has been demonstrated that the v e h i c l e continues to f i l l pores l a r g e r than the wavelength of l i g h t f o r 5 to 15 minutes a f t e r p r i n t i n g . Using r a d i o a c t i v e t r a c e r s i t has a l s o been shown that the o i l continues to migrate f o r two to three weeks a f t e r p r i n t i n g , f i n a l l y penetrating to approximately three quarters of the thickness of the newsprint paper s t u d i e d ^ . Since the surface energy of the o i l v e h i c l e i s u s u a l l y lower than that of the paper, i t can be presumed that t h i s long-term migration i s due to spreading. Thus, paper p r i n t ed with non-drying inks must have a high surface area i n order to separate the o i l phase from the pigment without the o i l penetrati n g to the other side of the paper (which would cause show through of the image due to opacity l o s s i n the paper). On the other hand, i f the c a p i l l a r y s t r u c t u r e of the surface of the paper i s too f i n e the rate of c a p i l l a r y i m b i b i t i o n of the o i l v e h i c l e w i l l be too slow causing set o f f of the f r e s h ink onto the p r i n t i n g press and onto i n a p p r o p r i a t e paper s u r f a c e s . P r i n t i n g with inks which dry by solvent evaporation or by p o l y m e r i z a t i o n can be done on very smooth papers since rapid v e h i c l e absorption i s not required to set the ink. In fact, uniform ink holdout i s g e n e r a l l y a prime r e q u i s i t e f o r p r i n t i n g with drying i n k s . Coating paper with mineral pigments i n a s u i t able binder i s a common way i n which to provide a smooth and less porous surface f o r p r i n t i n g . The mean pore s i z e i n the surface of coated paper i s about one order of magnitude smaller than that of the uncoated paper-. Ink penetration during impression i s thereby decreased. Thus, inks which set by solvent evaporation (heat s e t ) or by polymerization must be used on coated papers i n order to avoid set o f f and smearing. O f f s e t lithography i n v o l v e s a f u r t h e r complication of the model depicted i n Figure 1. The image i s o f f s e t from the p r i n t ing p l a t e to a corded rubber blanket which then t r a n s f e r s the image to paper i n the p r i n t i n g n i p . More importantly, f o u n t a i n s o l u t i o n , commonly c o n s i s t i n g of gum arable i n aqueous s o l u t i o n , i s a p p l i e d to the nonimage areas of the p l a t e and f i n d s i t s way onto the paper surface along with the ink. Fountain s o l u t i o n i s a l s o e m u l s i f i e d i n the ink which changes i t s r h e o l o g i c a l and wett i n g p r o p e r t i e s ^ ^. The paper must then absorb both an aqueous s o l u t i o n and an o i l - b a s e d ink with a delay that does not exceed the time f o r the paper to pass from one colour p r i n t i n g u n i t to another. In t h i s review, ink-paper i n t e r a c t i o n s during and a f t e r the p r i n t i n g nip w i l l be examined s t a r t i n g with the l e a s t complicated


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process, l e t t e r p r e s s , and then dealing with s p e c i f i c i n t e r a c t i o n s p e c u l i a r to the o f f s e t process.


LETTERPRESS PRINTING Perhaps the simplest form of p r i n t i n g i s the t r a n s f e r of an o i l - c a r b o n black ink from an unscreened p r i n t i n g plate to porous paper. Walker and Fetsko- were among the f i r s t to mathematicall y model t h i s process. They considered ink t r a n s f e r to be i n f l u enced by three f a c t o r s : contact between the ink f i l m on the p l a t e and the surface of the paper i n the p r i n t i n g n i p , immobiliz a t i o n of the ink i n the surface pores of the paper, and s p l i t t i n g of the remaining p o r t i o n of the ink f i l m as the p l a t e leaves the paper at the e x i t of the n i p . Figure 3 shows a t y p i c a l ink t r a n s f e r curve f o r newsprint when the amount of ink on the p r i n t ing p l a t e i s v a r i e d over a broad range. The i n i t i a l d i p i n the t r a n s f e r curve i s due to i n c r e a s i n g contact between ink and paper as the ink f i l m thickness i s i n creased and p r o g r e s s i v e l y shallower pores i n the surface of the paper are bottomed. As the amount of ink on the plate approaches commercial l e v e l s (about 5 g/m ) a maximum i n the f r a c t i o n of ink t r a n s f e r r e d from plate to paper i s reached. The f r a c t i o n a l ink t r a n s f e r exceeds 50 percent at t h i s point due to the ink immobilized i n surface c a p i l l a r i e s . This i s shown g r a p h i c a l l y i n Figure 4 where the f r a c t i o n f of the remaining or ' f r e e ' ink f i l m which s p l i t s and stays with the paper i s c a l c u l a t e d . The free ink f i l m s p l i t i s i n v a r i a b l y much l e s s than 50 percent at normal p r i n t i n g speeds. Thus as the amount of ink on the p r i n t i n g p l a t e reaches a l e v e l where the surface pores begin to be s a t i s f i e d the f r e e ink f i l m s p l i t predominates and the f r a c t i o n of ink t r a n s ferred declines. 2

These concepts t r a n s f e r equation:

are

modelled

in

the

Walker-Fetsko

y = A [bB + f ( x - bB)]

where:

ink

(1)

—kx 1 - e * ( f r a c t i o n of contact) 1 - e" ( f r a c t i o n of immobilized) amount of ink t r a n s f e r r e d to paper amount of ink on p r i n t i n g plate rate at which contact i s achieved with i n c r e a s i n g ink on p l a t e b • maximum amount of ink that can be immobilized i n the paper f - f r a c t i o n of free ink f i l m that s p l i t s and stays with the paper.

A B y x k

• = • • =

x / b



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Ink-Pa per Interactions

x-INK WEIGHT ON PLATE (g/m ) 2

Figure 3. Typical ink transfer (top) and fractional ink transfer (bottom) curves for newsprint printed at 4.6 m/s and 15.4 kN/m in a GFL rotary press ($).

Figure 4. Splitting of free inkfilmin printing nip. (F — fraction of free ink film split that stays with paper.)


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The equation i s f i t t e d to the f r a c t i o n a l ink t r a n s f e r curve i n order to maintain r e s i d u a l e r r o r homogeneous over the whole range of ink weights on the p r i n t i n g plate**. The estimates obtained f o r the parameters k, b, and f r e f l e c t fundamental p r i n t i n g c h a r a c t e r i s t i c s of the paper. For example, as shown i n F i g ure 5 the k parameter i s a measure of the smoothness of the paper under p r i n t i n g c o n d i t i o n s . By d i f f e r e n t i a t i n g the Walker-Fetsko equation with respect to each of the parameters and m u l t i p l y i n g the d e r i v a t i v e by one standard d e v i a t i o n about the mean value of these parameters f o r 29 d i f f e r e n t p r i n t i n g s of newsprint Mangin et a l showed the i n f l u e n c e paper c h a r a c t e r i s t i c s can have on ink t r a n s f e r . As shown i n Figure 6 the main i n f l u e n c e of the smoothness parameter k i s at lower ink weights. The i n f l u e n c e of the p o r o s i t y r e l a t e d parameter b peaks and then d e c l i n e s as the s p l i t parameter f begins to predominate. At commercial ink weights of about 5 g/m the smoothness and p o r o s i t y parameters are e q u a l l y important while the s p l i t parameter ( r e l a t e d to the a f f i n i t y of the paper to ink and ink rheology) cannot be neglected.


8

2

Contact In 1947, Chapman- developed a instrument to measure the o p t i c a l contact between a f l a t glass prism and the surface of paper. The idea of measuring the smoothness of paper under pressure s i m u l a t i n g the compression of paper i n a p r i n t i n g nip has been explored by s e v e r a l authors s i n c e — " - ^ . Perhaps the most s t r i k i n g r e s u l t of t h i s work i s the low f r a c t i o n of contact that i s achieved. Under p r i n t i n g compression o p t i c a l contact with smooth newsprint i s about 20 percent and seldom above 50 percent f o r coated papers. C l e a r l y , the deformation of the f l u i d ink i n the p r i n t i n g nip must r a i s e the t o t a l contact considerably. One way of c a l c u l a t i n g the e f f e c t i v e contact between ink and paper i s to measure the f r a c t i o n of the paper surface covered with ink when p r i n t i n g with a s o l i d p r i n t i n g form. Using a black ink i t i s p o s s i b l e to c a l c u l a t e the f r a c t i o n a l coverage from the r e f l e c tances of the p r i n t e d and unprinted paper: Rp where:

Rp Roo C

= CRj +

(1 - C)

R^

r e f l e c t a n c e from printed paper r e f l e c t a n c e from unprinted paper r e f l e c t a n c e of ink f r a c t i o n a l ink coverage


(2)


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X (INK ON PLATE)

Figure 5. Effect of paper smoothness on contact fraction.

0.15 •

x - Ink weight on plate(g nV) Figure 6. Change in fractional ink transfer with a shift in parameters k, h, and f in the Walker-Fetsko equation equal to one standard deviation about their mean fitted values for 29 printings of newsprint ($).


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Assuming the r e f l e c t a n c e of the ink i s approximately equal to zero ( i . e . n e g l e c t i n g translucence of the pigment and g l o s s ) : = 0 then

C = 1 - *E

(3)

This i s the b a s i s of the Larocque l e t t e r p r e s s q u a l i t y t e s t * ^ wherein 2 g/m of ink i s used on the p r i n t i n g p l a t e . At t h i s ink l e v e l the f r a c t i o n a l coverage v a r i e s from 0.3 to 0.6 f o r rough and smooth newsprint, r e s p e c t i v e l y .


2

Even at 0.5 f r a c t i o n a l coverage the r e s u l t i n g p r i n t appears v i s u a l l y to be a r e l a t i v e l y uniform gray at reading d i s t a n c e . For j u s t - n o t i c e a b l e d i f f e r e n c e s i n contrast the r e s o l u t i o n of the human eye at t h i s distance i s about 100 um. Much of the noncont a c t area i n a s o l i d p r i n t i s f i n e r than t h i s r e s o l u t i o n l i m i t and i s thus i n t e g r a t e d to gray by the eye. I t should a l s o be mentioned that the area covered by ink i s overestimated by equat i o n (3) and by the eye due to a reduction i n the r e f l e c t a n c e from the unprinted p o r t i o n s of the p r i n t e d s o l i d caused by the shadow e f f e c t of adjacent inked area ( i . e . the l i g h t s c a t t e r e d back from the unprinted areas i s less than that from an unprinted sheet of the same paper). T h i s o p t i c a l spreading phenomenon i s commonly known as the Y u l e - N e i l s e n e f f e c t — . L a t e r a l spreading of ink i n the p r i n t i n g n i p i s a l s o a f a c t o r i n s o l i d coverage and i n h a l f t o n e dot reproduction. In f a c t , O i t t i n e n ^ has c a l c u l a t e d the r e l a t i v e e f f e c t s of p h y s i c a l spreading and o p t i c a l spreading on the modulation t r a n s f e r funct i o n f o r h a l f t o n e p r i n t i n g and found the former to be g e n e r a l l y more s i g n i f i c a n t . The equation used to c a l c u l a t e l a t e r a l spread of ink during impression was that of Dienes and Klemm— f o r flow between p a r a l l e l plates:

n where:

(*)

d = dot diameter P = p r i n t i n g pressure x = ink f i l m thickness t dwell time under pressure n = viscosity

Immobilization

and

From the f o r e g o i n g , i t i s c l e a r that the contact between ink paper depends on the dynamic c o m p r e s s i b i l i t y of the paper.


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The void volume and the d i s t r i b u t i o n of pore s i z e s a l s o change with compression of the paper a f f e c t i n g the h y d r a u l i c impression of ink during impression. C o l l e y and P e e l ^ proposed the f o l l o w i n g master t i o n to d e s c r i b e the compression of paper:

creep equa-

AL = A (1 - tanh u)


u = a log P

+ 3 l o g t + y M + cx8 + e

m a x

where AL^ = compression L

o

A

>

P

a

f 3, Y, a and e are e m p i r i c a l l y derived

m a x

parameters.

= maximum pressure

t = dwell time at P „ „ max

v

M = moisture content of paper 6 = temperature of paper The equivalence f o r creep compression of a l t e r i n g pressure, dwell time, moisture, or temperature i s e x p l i c i t y expressed i n the argument u. H s u — made a m o d i f i c a t i o n of the Kozeny equation using a simpler compression model to describe the h y d r a u l i c impression of ink i n t o paper under p r i n t i n g compression. Knowing the void f r a c t i o n and e f f e c t i v e c a p i l l a r y radius f o r newsprint from mercury i n t r u s i o n measurements, the void f r a c t i o n and e f f e c t i v e c a p i l l a r y radius under p r i n t compression can be c a l c u l a t e d using Equation (5) with the e m p i r i c a l parameters derived by C o l l e y and P e e l . The void f r a c t i o n and e f f e c t i v e c a p i l l a r y radius under p r i n t i n g compression can a l s o be measured v i a a s p e c i a l l y cons t r u c t e d mercury i n t r u s i o n apparatus.In e i t h e r case, these values can be used to c a l c u l a t e the volume per u n i t area of ink impressed i n t o paper during i t s passage through the p r i n t i n g nip assuming P o i s e u i l l e flow i n t o a s t r u c t u r e having a void f r a c t i o n made up of c y l i n d r i c a l c a p i l l a r i e s which are tortuous but not interconnected:


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where: V_ ^ = mean depth of p e n e t r a t i o n of ink i n t o paper e = void f r a c t i o n of paper under p r i n t i n g r

= effective capillary compression

compression

radius of paper under

printing

t » time under pressure


n = v i s c o s i t y of ink P = printing

pressure

T = tortuosity (actual length length of c a p i l l a r y ) .

of

capillary/projected

According to equation (5) and using the parameters reported by C o l l e y and P e e l , the void f r a c t i o n of newsprint under l e t t e r press p r i n t i n g pressures of 2.5 MPa f o r 1 ms (approximate f i g u r e s based on e f f e c t i v e height and width of the nip pressure pulse) w i l l be reduced from 0.6 f o r the uncompressed state to 0.48 ( a s suming e q u i l i b r i u m with a standard temperature and humidity atmosphere of 23°C and 50% RH). I f the e f f e c t i v e pore radius f o r newsprint was 2 um before compression, i t w i l l be 1.75 um under compression. Taking a v i s c o s i t y of 2 Pa.s and a surface tension of 30 mN/m f o r the ink and a t o r t u o s i t y f a c t o r of 5 f o r newsprint, equation (6) p r e d i c t s a h y d r a u l i c impression of 3 um f o r the above p r i n t i n g c o n d i t i o n s . T h i s agrees reasonably w e l l with the amount of ink t r a n s f e r r e d to newsprint i n commercial l e t t e r p r e s s printing. Since, as shown i n Figure 1, the dynamic contact angle i s greater than 90 degrees during impression, c a p i l l a r y i m b i b i t i o n may be neglected as a c o n t r i b u t o r to ink t r a n s f e r . Under the same p r i n t i n g c o n d i t i o n s and taking the e f f e c t i v e pore r a d i u s , void f r a c t i o n , and t o r t u o s i t y of a coated paper to be about 0.2 um, 0.3 and 5, r e s p e c t i v e l y , the c a l c u l a t e d hydraul i c impression becomes 0.2 um. Thus, f o r coated papers the t o t a l amount of ink impressed i s a small f r a c t i o n of the amount of ink t r a n s f e r r e d to the paper during p r i n t i n g . In other words, the i m m o b i l i z a t i o n of ink i s predominant i n the t r a n s f e r of ink to newsprint, while the free ink f i l m s p l i t i s predominant i n the case of coated papers. A p r a c t i c a l i m p l i c a t i o n of these c a l c u l a t i o n s i s shown i n F i g u r e 7. The o p t i c a l d e n s i t y (blackness) of a s o l i d p r i n t i s p l o t t e d as a f u n c t i o n of the amount of ink on three types of paper and a non-porous p l a s t i c f i l m (mylar). The l e s s porous the



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Plastic film

Cast-coated Coated Newsprint

/ I 0

i 1

i 2

i 3

i 4

i 5

i 6 2

Ink weight on sample (g/m ) Figure 7.

Optical density (blackness) as a function of the amount of ink on paper or mylarfilm(48j.


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paper, the l e s s ink i s impressed i n t o the paper. Ink holdout r e s u l t s i n a greater o p t i c a l density at a given weight of ink on the paper. Ink rheology a f f e c t s the ink immobilization, l a t e r a l spreadi n g , and ink f i l m s p l i t t i n g components of ink t r a n s f e r . More p s e u d o p l a s t i c inks have lower v i s c o s i t i e s during impression, and as p r e d i c t e d by equations (4) and (6) t h i s r e s u l t s i n greater l a t e r a l coverage and higher immobilization. Ink elasticity ( r a t i o of storage modulus to dynamic v i s c o s i t y ) has a l s o been shown to improve coverage and t r a n s f e r — . F i n a l l y ink shortness ( r a t i o of apparent y i e l d s t r e s s to p l a s t i c v i s c o s i t y ) has been found to a f f e c t t r a n s f e r — ^5 and l a t e r a l coverage (halftone dot sharpness)^. Presumably, the filaments of shorter inks f r a c t u r e c l o s e r to the nip center leading to a more uniform t r a n s f e r .

Free Ink Film S p l i t The mechanism of ink immobilization i s an important f a c t o r i n determining the amount that w i l l be free to s p l i t at the e x i t of the p r i n t i n g n i p . As shown i n Figure 2, the p o s i t i v e pressure generated i n the nip i s much greater than the negative pressure pulse which f o l l o w s i t . H y d r a u l i c impression i n t o i n t e r f i b r e c a p i l l a r i e s i n paper against the viscous r e s i s t a n c e of the ink i s not s i g n i f i c a n t l y reversed during ink f i l m s p l i t t i n g . Thus, the impressed ink can be considered immobilized with respect to the ink f i l m s p l i t . Note that the ink does not need to wet the surface of the pores f o r t h i s form of immobilization. However, o p t i c a l contact measurements show that there i s a s i g n i f i c a n t p o r t i o n of the surface of any paper when compressed that l i e s p a r a l l e l to the surface of the p r i n t i n g p l a t e . Here wetting and adhesion of the ink to the paper must occur f o r the ink to be t r a n s f e r r e d since ink cannot be immobilized i n these regions by h y d r a u l i c impression. I t i s a l s o apparent that the smoother the surface of the paper the more important wetting and adhesion becomes f o r good t r a n s f e r . DeGrSce and M a n g i n — have treated the case of ink t r a n s f e r to nonporous model substrates such an mylar. The surface of mylar contains p i t s of about 1 Mm depth. Very l i t t l e void volume e x i s t s f o r ink i m m o b i l i z a t i o n , thus allowing ink f i l m s p l i t t i n g to be examined i n i s o l a t i o n . The t r a n s f e r of two ink v e h i c l e o i l s of d i f f e r i n g v i s c o s i t i e s and corresponding inks to mylar i s shown i n Figure 8 as a f u n c t i o n of ink weight on the p r i n t i n g p l a t e and p r i n t i n g speed i n a r o t a r y l e t t e r p r e s s u n i t using a 100 percent tone p l a t e . The f r a c t i o n of ink t r a n s f e r r e d to the mylar decreases with ink weight on the p l a t e and as the p r i n t i n g speed or v i s c o s i t y i s increased and can be as low as 0.25 at commercial p r i n t i n g speeds.




399


20.

Key: LV oil, low viscosity oil; HV oil, high viscosity oil; LV-12% C ink, LV oil with 12% by weight of carbon black pigment; and HV-12% C ink, HV oil with 12% by weight of carbon black pigment (21).


400

REPROGRAPHIC

TECHNOLOGY


The c h e m i s t r i e s of commercial o i l v e h i c l e s vary with respect to the r a t i o of aromatic, p a r a f f i n i c and c y c l o p a r a f f i n i c carbon ( g e n e r a l l y about 20, 40, and 40 percent, r e s p e c t i v e l y ) , degree of o x i d a t i o n , and amount and character of i m p u r i t i e s . The surface energies of the high and low v i s c o s i t y ink v e h i c l e s shown i n F i g ure 8 were 31.3, and 26.0 mN/m, r e s p e c t i v e l y (as measured by the Du Nolly r i n g method). The v i s c o s i t i e s of the o i l s and the o i l s mixed with 12 percent carbon black pigment are shown i n Figure 9 as a f u n c t i o n of the rate of shear i n a cone-plate v i s c o m e t e r — . In t h i s range of shear rates the o i l s are Newtonian f l u i d s while the corresponding inks are markedly p s e u d o p l a s t i c . As the pigment i n the inks i s broken down to p r o g r e s s i v e l y f i n e r agglomerates at i n c r e a s i n g r a t e s of shear the v i s c o s i t i e s of the inks approach those of t h e i r r e s p e c t i v e o i l v e h i c l e s . Thus, at the high rates of shear encountered i n the p r i n t i n g nip the v i s c o s i t i e s of the o i l s and the corresponding inks can be expected to be s i m i l a r . On a fundamental l e v e l , wetting and adhesion should be improved by decreasing v i s c o s i t y and/or surface t e n s i o n . Thus, the lower v i s c o s i t y , lower surface tension o i l and corresponding ink should wet and adhere to the mylar better than the higher v i s c o s i t y o i l and i n k . Zisman— has a l s o pointed out that a i r bubbles trapped between a l i q u i d and a s p e r i t i e s i n the surface of impervious m a t e r i a l s l i k e mylar can s e r i o u s l y i n h i b i t adhesion and cause a G r i f f i t h f r a c t u r e when the adhesive bond i s s t r e s s e d . Thus, at the outgoing s i d e of the p r i n t i n g n i p , nonuniform adhesive f a i l u r e between the ink and the mylar could r e s u l t i n l e s s ink t r a n s f e r . This e f f e c t could be expected to become more pronounced with greater p r i n t i n g speed, and higher v i s c o s i t y and higher surface tension of the o i l or i n k . However, DeGrice and Mangin^i observed that the ink t o t a l l y covered the surface of the mylar even at the higher press speeds (5.9 m/s), and thus concluded that wetting and adhesion occurred uniformly over the surface of the mylar. F i l a m e n t a t i o n of the ink f i l m occurs as the r e s u l t of c a v i t a t i o n i n response to sub-atmospheric pressures that occur beyond the centre of the p r i n t i n g nip (see Figures 1 and 2). DeGr£ce and M a n g i n — have pointed to asymmetric c a v i t a t i o n favouring the s u b s t r a t e surface as the cause of ink filaments forming c l o s e r to the paper surface than the surface of the p r i n t i n g p l a t e . They argue that the surfaces of paper and mylar provide more nucleat i o n s i t e s f o r c a v i t a t i o n than e x i s t i n the bulk of the o i l or ink. A i r entrained i n the p r i n t i n g nip could a l s o provide s i t e s f o r c a v i t a t i o n near the surface of the s u b s t r a t e . Ink filaments thus form c l o s e r to the substrate than the p l a t e - an e f f e c t which becomes more pronounced with t h i c k e r ink f i l m s . Filament flow tends to equalize the asymmetric p o s i t i o n of the f i l a m e n t . I n c r e a s i n g the p r i n t i n g speed reduces the time f o r filament flow



L Y N E AND ASPLER


401

Figure 9. Viscosity of letterpress ink and oils (described in Figure 8) measured as a function of shear rate in a Rheometrics Mechanical Spectrometer with cone and plate fixtures and a cone angle of 2°20' (22).


402

REPROGRAPHIC

TECHNOLOGY

before rupture and thus, i n c r e a s i n g p r i n t i n g speed or v i s c o s i t y w i l l r e s u l t i n filament s p l i t t i n g c l o s e r to the mylar or paper surface and decreased ink t r a n s f e r . C a v i t a t i o n , f i l a m e n t a t i o n , v i s c o e l a s t i c i t y and tack w i l l be discussed f u r t h e r i n connection with l i t h o g r a p h i c p r i n t i n g .


P o s t - T r a n s f e r Phenomena Paper relaxes v i s c o e l a s t i c a l l y f o l l o w i n g the post-nip decomp r e s s i o n . This recovery of void volume r e s u l t s i n a s p i r a t i o n of ink i n t o expanding c a p i l l a r i e s i n the paper. This may be the l a s t e f f e c t i v e movement of the whole ink - t h e r e a f t e r the v e h i c l e migrates i n c r e a s i n g the pigment concentration and l e a v i n g the pigment d i s t r i b u t e d e x p o n e n t i a l l y through the thickness of the paper . 2

Three mechanisms can be considered f o r the migration of an ink v e h i c l e such as o i l : c a p i l l a r y i m b i b i t i o n , spreading, and bulk d i f f u s i o n . I t takes about 100 ms f o r a f r e s h p r i n t to t r a v e l from one nip of the p r i n t i n g press to the next i n a comm e r c i a l m u l t i c o l o r p r i n t i n g press. In t h i s time the o i l v e h i c l e i s s u f f i c i e n t l y drained from the ink that set o f f does not occur i n the second colour u n i t . The d r i v i n g f o r c e f o r c a p i l l a r y i m b i b i t i o n i s surface t e n s i o n : p

where:

P Y r 6

= = = =

2y COS 6 r

=

(7)

c a p i l l a r y pressure surface tension of v e h i c l e e f f e c t i v e c a p i l l a r y radius contact angle between v e h i c l e and c a p i l l a r y w a l l s .

Assuming that the advancing contact angle f o r o i l on the c a p i l l a r y walls i s approximately zero equation (7) may be combined with equation (6) to y i e l d : V

A

= e_ / T Y F T \2n

(8)

I f the o i l v e h i c l e has a surface tension of 30 mN/m and a v i s c o s i t y of 0.5 Pa.s and i f the paper i s uncompressed newsprint the void f r a c t i o n , t o r t u o s i t y and mean e f f e c t i v e c a p i l l a r y radius w i l l be approximately 0.6, 5 and 2 um, r e s p e c t i v e l y . Equation (8) then p r e d i c t s that i n 100 ms c a p i l l a r y i m b i b i t i o n could withdraw 9 um of v e h i c l e from the s u r f a c e .


20.

L Y N E AND ASPLER

403



T h i s i s more than adequate to set news inks at commercial l e v e l s of i n k a p p l i c a t i o n . C a p i l l a r y i m b i b i t i o n i s thus the predominant ink s e t t i n g mechanism since spreading and bulk d i f f u s i o n are much slower processes. However, two complications a r i s e i n t h i s s i m p l i s t i c model. I m p l i c i t i n the model i s the assumption that the c a p i l l a r i e s are connected to an i n e x h a u s t i b l e supply of l i q u i d . This i s not the case, and i t would be more reasonable to assume that as the l a r g e r c a p i l l a r i e s d r a i n the o i l from the surface of the paper d i f f e r e n t i a l c a p i l l a r y pressures i n the interconnected network d r a i n the v e h i c l e i n t o p r o g r e s s i v e l y smaller c a p i l l a r i e s emptying the l a r g e r ones (see Figure 1 ) . These d i f f e r e n t i a l pressures can be expressed as: AP = 2 y cos 6 ( I - I ) r • r where r* i s the radius of the smaller

(9)

capillary.

Secondly, the concept of c a p i l l a r y i m b i b i t i o n being respons i b l e f o r the s e t t i n g of an ink by d r a i n i n g the v e h i c l e away from the pigment can be disputed. Larsson and S u n n e r b e r g used r a d i o a c t i v e t r a c e r s to monitor pigment concentration i n news inks a f t e r the p r i n t i n g nip. The average pigment concentration rose from 14.4 percent to 25.5 percent i n one hour and reached e q u i l i brium a f t e r 24 hours. The average pigment concentration did not r i s e a p p r e c i a b l y i n the f i r s t second a f t e r p r i n t i n g suggesting that whole i n k may be imbibed i n i t i a l l y a f t e r p r i n t i n g . The s e t t i n g e f f e c t may occur at the surface of paper where pigment i s l e f t behind as the whole ink i s imbibed. This kind of chromatographic e f f e c t i s more reasonable than drainage by c a p i l l a r y i m b i b i t i o n because at some point during the concentration of the pigment c a p i l l a r i e s w i l l be created i n the concentrated ink that are f i n e r than those i n the paper. Thus, according to equation (9) the v e h i c l e w i l l thenceforth be retained by the pigment. 26

The d e n d r i t i c s t r u c t u r e portrayed i n Figure 1 i s not v a l i d for the s t r u c t u r e of paper c o a t i n g s . The more r e g u l a r packing of c l a y p l a t e l e t s , or calcium carbonate p a r t i c l e s probably leads to a s t r u c t u r e c o n s i s t i n g of l a r g e r voids joined by smaller i n t e r stices. Equation (9) p r e d i c t s that i n k cannot penetrate by c a p i l l a r y i m b i b i t i o n beyond the f i r s t i n t e r s t i c i a l v o i d , thus e f f e c t i v e l y reducing the void volume and rate of p e n e t r a t i o n . F o r t h i s reason inks which set by solvent evaporation and/or polymeri z a t i o n are g e n e r a l l y used on coated papers instead of penetrating inks. According to equation (8) the rate of p e n e t r a t i o n of ink v e h i c l e i n t o newsprints observed by Lyne and Madsen- 10 to 100 seconds a f t e r p r i n t i n g of 0.1 um/s corresponds to c a p i l l a r i e s having a mean e f f e c t i v e radius of about 0.2 um. This agrees w e l l


404

REPROGRAPHIC

TECHNOLOGY

with the observation that the opacity of paper continues to decrease f o r a few minutes a f t e r p r i n t i n g — s i n c e pores of 0.2 um radius and l a r g e r would s c a t t e r l i g h t u n t i l f i l l e d by the ink vehicle. Bulk d i f f u s i o n of o i l v e h i c l e i n t o the c e l l walls of f i b r e s i s a very slow mechanism. The h a l f time f o r d i f f u s i o n of an o i l of about C through a s i n g l e f i b r e w a l l of a glassy p o l y mer such as c e l l u l o s e at room temperature would be i n the order of weeks. This leaves spreading of the low surface energy o i l v e h i c l e ( c i r c a 30 mN/m) over the surface of the f i b r e s as a reasonable long-terra mechanism f o r o i l migration i n paper. The surface of mechanically prepared wood f i b r e s i s p r i m a r i l y l i g n i n while bleached chemically pulped f i b r e has a c e l l u l o s i c s u r f a c e . Surface energies f o r various forms of l i g n i n and c e l l u l o s e are l i s t e d i n Table I along with t h e i r London components. The surface energy of paper coatings v a r i e s p r i n c i p a l l y with the binder used, n a t u r a l binders being more polar than l a t e x e s .


3 0

Table I.

Surface Free Energies of Paper and Related Y

Y

Substrates

L

mN/m Crystalline cellulose Thermomechanical pulp f i b r e s Holocellulose F a t t y - a c i d surface Mylar poly(ethylene t e r e p h t h a l a t e ) Coated Paper Regenerated (low c r y s t a l l i n i t y ) cellulose Lignin Water

Using values from Table Young's spreading c o e f f i c i e n t : S = Y

2

= w W

A

=

2

h

(y\yb

W

c

where:

I

- Y

it

" Y

2

Reference

30-45 68.7

48 37 48 24 40 30 38-42

52.5 72

36-43 22

is

possible

(27) (27) (28,29) (47) (45) (44) (28) (30)

to

calculate

(10)

1 2

- W

A

(11)

c

+

2

= 2Y

%

2

W^ = work of adhesion WQ = work of cohesion


(12) (13)

20.

L Y N E AND ASPLER


405

Neglecting the polar components for the case of o i l on l i g n i n S i s + 10 and f o r o i l on c e l l u l o s e i s + 16. These values are s u f f i c i e n t l y large that conditions f o r spreading are h i g h l y favourable. However, i f a l l the o i l t r a n s f e r r e d to paper spread to a monolayer i t would more than cover the t o t a l surface area of newsprint ( c i r c a 1 m /g). Thus, o i l must a l s o be retained i n the f i n e r c a p i l l a r i e s i n the paper and i n the ink pigment.


2

F i n a l l y , three important f a c t o r s i n p r i n t q u a l i t y - p r i n t d e n s i t y , show through, and set o f f are graphed i n Figure 10 as a f u n c t i o n of the amount of ink on the p r i n t e d paper. The p r i n t e r g e n e r a l l y c o n t r o l s the amount of ink on the p r i n t i n g p l a t e i n order to a r r i v e at a predetermined p r i n t density, or contrast i n the p r i n t e d image. Papers of d i f f e r i n g smoothness, p o r o s i t y , and a f f i n i t y to ink w i l l e x h i b i t considerable v a r i a t i o n i n the r e l a t i o n s h i p s among these three p r o p e r t i e s . I d e a l l y , a paper should give a p r i n t density of 1.0 at as low an ink l e v e l as p o s s i b l e i n order to avoid show through and set o f f problems.

OFFSET LITHOGRAPHY With respect to the ink-paper i n t e r a c t i o n s two major f a c t o r s separate o f f s e t lithography from l e t t e r p r e s s : f o u n t a i n s o l u t i o n i s t r a n s f e r r e d to paper i n nonimage areas and to paper i n image areas by e m u l s i f i c a t i o n i n the ink, and, since the rubber blanket used i n o f f s e t creates c l o s e contact with the paper surface, thinner and more h i g h l y pigmented ink f i l m s can be a p p l i e d to paper, a i d i n g ink holdout. Thus, paper f o r the o f f s e t process must wet r a p i d l y with polar fountain s o l u t i o n as w e l l as o i l based inks, and must withstand ink f i l m s p l i t t i n g forces without l i n t i n g , p i c k i n g , or delaminating. Excessive absorption of fount a i n s o l u t i o n may r e s u l t i n rupture of i n t e r f i b r e hydrogen bonds l i b e r a t i n g f i b r e s and d e b r i s , and i t may soften coating binders l e a d i n g to p i c k i n g . I f the pH of the paper i s too low, a c i d s o l u b i l i z a t i o n by the s o r p t i o n of fountain s o l u t i o n w i l l cause i n t e r f e r e n c e with ink d r y i n g . The h y g r o - s t a b i l i t y of the paper and i t s t e n s i l e strength when moistened with fountain s o l u t i o n can a l s o pose press r u n n a b i l i t y problems. Nonabsorption of fount a i n s o l u t i o n t r a n s f e r r e d to the paper i n the nonimage area of one colour u n i t can cause r e f u s a l of the ink to t r a n s f e r to the paper i n the next colour p r i n t i n g u n i t . This i s i l l u s t r a t e d i n Figure 11 f o r varying times between the a p p l i c a t i o n of a water f i l m to paper and o v e r p r i n t i n g with o i l - b a s e d ink.

Dynamic Wetting of Paper by Fountain S o l u t i o n B r i s t o w ^ has described a device f o r measuring the s o r p t i o n of various l i q u i d s i n t o paper. As shown i n Figure 12 a paper s t r i p i s mounted on a r o t a t i n g wheel, and i s drawn past a minia-


406

REPROGRAPHIC

SO

ST

•

A

1.0r 0.12r

TECHNOLOGY

PD •

1 1r

0.8


0.6 -

0.4

0.2 -

0

L

0.02 L 0.6

X - INK WEIGHT ON SAMPLE (g/m ) 2

Figure 10.

Print density (%), show through (A) and set off ( • ) as a function of the amount of ink on the printed sample (48,).

Figure 11. The effect of altering time between the application of water to the surface of a coated board and overprinting with a lithographic ink. (Reproduced, with permission, from Ref. 31. Copyright, Marcel Dekker, Inc.)


20.

L Y N E AND ASPLER


407

ture d i s t r i b u t o r (or open headbox) which i s deadweighted so that i t r e s t s on the paper s t r i p with a pressure of 0.1 MPa. The d i s t r i b u t o r i s f i l l e d with a known amount of l i q u i d which has a time a v a i l a b l e f o r s o r p t i o n determined by the p e r i p h e r a l speed of the wheel V and the width of the s l i c e opening (L = 1 mm) i n the distributor. Since t h i s speed may be v a r i e d , the r e l a t i o n s h i p between the amount of l i q u i d t r a n s f e r r e d per unit area of paper and the time a v a i l a b l e f o r s o r p t i o n may be e s t a b l i s h e d as i n F i g ure 13.


The amount described by:

of l i q u i d

which

A = K

r

t r a n s f e r s to the paper

+ K

a

th

can

be

(14)

where K i s the volumetric surface roughness, or the amountof l i q u i d which would t h e o r e t i c a l l y run i n t o the surface topography of paper at zero time, and K i s an a b s o r p t i o n c o e f f i c i e n t . K i s a l s o the mean depth to which the f l u i d may penetrate before c a p i l l a r y f o r c e s are i n i t i a t e d . As shown i n Figure 13, there i s a wetting delay before water begins to be sorbed i n t o interfibre capillaries. The wetting delay f o r mineral o i l i s too short to be resolved by the Bristow instrument. r

a

r

The wetting and spreading of high v i s c o s i t y f l u i d s on paper i s a f f e c t e d by surface r o u g h n e s s — . Hoy land et a l . - ^ - found that the wetting delay f o r water was unaffected by the surface roughness of bleached k r a f t paper, but that i n c r e a s i n g the v i s c o s i t y of water with s t a r c h brought about a roughness e f f e c t on the wetting delay. A porous, pure cotton c e l l u l o s e paper such as Whatman chromatographic paper does not show a wetting delay to water. However, as shown i n Figure 14 s i z i n g the Whatman paper i n a 2 percent isopropanol s o l u t i o n of Dupont Quilon-C causes a wetting delay of about 80 ms and a severe r e d u c t i o n i n the rate of a b s o r p t i o n , K , d e s p i t e the f a c t that the mean pore size measured by mercury i n t r u s i o n does not change as the r e s u l t of the s i z i n g . Quilon-C creates a f a t t y a c i d b a r r i e r to wetting i n much the same way as n a t u r a l l y occuring f a t t y a c i d s i n wood create b a r r i e r s to wetting of the k r a f t l i n e r shown i n Figure 13 and newsprint as shown i n Figure 15. a

The r e s u l t i n g wetting delay i s exacerbated by aging or heati n g ^ •^ . As shown i n Figure 15 the newsprint manufactured from f r e s h pulp has a wetting delay of 80 ms ( g e n e r a l l y the wetting delay f o r f r e s h newsprint l i e s between 40 and 80 ms) while newsp r i n t made from f l a s h d r i e d bale pulp aged f o r one year e x h i b i t s a wetting delay of about 0.5 seconds. T h i s progressive spreading of the r e s i n and f a t t y a c i d s with aging i s t h e r e f o r e r e f e r r e d to as s e l f - s i z i n g . t


408

REPROGRAPHIC T E C H N O L O G Y

TIME AVAILABLE FOR SORPTION =

^


HEADBOX

WPAPER

STRIP

ROTATING WHEEL

Figure 12. Principle of Bristow's instrument for dynamic wetting and absorption studies. (Reproduced, with permission, from Ref. 31. Copyright, Marcel Dekker, Inc.)

Liquid transferred to paper 2

(mL/m )

0.01

0.1

02

05

1-0

1.5

Time (sec) Figure 13.

Amount of liquid transferred to paper (kraft liner), on a square-root time axis (32). Key: O, oil-A; A , oil-B; and water.


20.

L Y N E AND ASPLER

409


400

r

300

-

Water transferred to paper 2


(mL/m )

200

100

4 20 80 200 400 8

4

0

800

2000

Time (ms)

Figure 14. Dynamic absorption of water by chromatographic papers (46). Key: •» Whatman #4 (coarse); A , Whatman #5 (fine); O , Whatman #/ (medium); and Whatman #1 (sized).

100

80

Water transferred to paper

60

2

(mL/m ) 40

20

ill I I 4 20 80 200 400 8

4

0

800

2000

Time (ms)

Figure 15. Wettability of newsprint made from fresh pulp (O) and from a flash dried bale pulp aged for one year (%) (33).


410

REPROGRAPHIC

TECHNOLOGY

Lyne Suranyi et al.2-±, and Takeyama and Gray 2^. have shown that solvent e x t r a c t i o n of f a t t y a c i d s has l i t t l e e f f e c t on wett i n g timeOn the other hand Suranyi et a l . ^ showed that corona discharge could reduce the wetting delay and Takeyama and Gray^found that a sodium methoxide e x t r a c t i o n was necessary to reduce the wetting delay of paper heated i n the presence of s t e a r i c acid. Thus, i t appears that with heating or aging the f a t t y a c i d s i n wood may become chemically bound to f i b r e surfaces i n paper.


1

The question remains as to whether uncontaminated c e l l u l o s e has a wetting delay to water. H o y l a n d — has reported a wetting delay of 21 ms f o r water p e n e t r a t i o n i n t o compacted pad of f u l l y bleached k r a f t f i b r e using a conductance probe technique. Using the Bristow apparatus on a pure, c l e a n , regenerated cellulose f i l m (cellophane) and on Whatman chromatographic paper which had been repulped, beaten to low freeness and then calendered h e a v i l y to avoid rapid i n t e r f i b r e s o r p t i o n the a u t h o r s — have demonstrated a wetting delay of about 8 ms on both s u r f a c e s . The authors have p o s t u l a t e d that the wetting delay i s the time required before l i q u i d - l i k e water i s absorbed onto f i b r e surfaces - a process which i s impeded by s i z i n g or s e l f s i z i n g ^ . After l i q u i d - l i k e water begins to be absorbed on f i b r e s u r f a c e s , bulk water can advance on the walls of i n t e r f i b r e c a p i l l a r i e s i n i t i a t i n g rapid c a p i l l a r y s o r p t i o n . The r a t e of s o r p t i o n through the porous, unsized Whatman papers i s so r a p i d that s a t u r a t i o n e f f e c t s can be seen as a downward curvature of the s o r p t i o n curves shown i n Figure 14. On the other hand, the s o r p t i o n curve f o r f r e s h newsprint shown i n F i g ure 15 i s curved upward. C a p i l l a r y s o r p t i o n i s slow enough i n the f i n e r pores of newsprint that s o r p t i o n of water i n t o f i b r e w a l l s and the r e s u l t i n g s w e l l i n g of the f i b r e s can s i g n i f i c a n t l y i n c r e a s e the s i z e of i n t e r f i b r e c a p i l l a r i e s during s o r p t i o n . T h i s increase i n the i n t e r f i b r e c a p i l l a r y s i z e can be seen as an i n c r e a s e i n the rate of s o r p t i o n with time (or upward curvature) for the conventional newsprint i n Figure 15. Can these r e s u l t s f o r water s o r p t i o n be r e l a t e d to the app l i c a t i o n of f o u n t a i n s o l u t i o n to paper during l i t h o g r a p h i c printing? In Figure 16 the a d d i t i o n of a s u r f a c t a n t i s seen to have l i t t l e e f f e c t on the dynamic surface tension of water at a s u r f a c e age of 3 ms. Thus, on the time s c a l e of the wetting of newsprint, f o u n t a i n s o l u t i o n s which c o n s i s t p r i n c i p a l l y of an aqueous s o l u t i o n of gum arable may behave s i m i l a r l y to the case of water alone. Conversely, as shown i n Figure 17 f o u n t a i n s o l u t i o n s c o n t a i n i n g isopropanol e x h i b i t lower dynamic surface tensions and should wet paper more r e a d i l y than water alone.



411



20.

2o I 0

I 5

i 10

I 15

I 20

I 25

i 30

Concentration (percent) Figure 17. Effect of isopropanol concentration on the static and dynamic surface tension of water (31).


412



The E f f e c t of pH on Ink

Drying

The e x t r a c t i v e pH of paper depends on manufacturing processes (e.g. the pulping method used, bleaching, papermachine a d d i t i v e s such as alum, s i z i n g agents, and c o a t i n g ) . Inks used i n sheet-fed o f f s e t lithography set and dry by the o x i d a t i o n i n i t i a t e d polymerization of l i n s e e d o i l or s y n t h e t i c polymer components. Metal s a l t s are used i n order to a c c e l e r a t e the rate of p o l y m e r i z a t i o n and thereby avoid set o f f and smearing of the f r e s h ink. As shown i n Figure 18 the a c t i o n of these c a t a l y s t s i s i n t e r f e r e d with i f the e x t r a c t i v e pH of the paper i s below about 5.5. To some degree the pH of the fountain s o l u t i o n can be varied to compensate f o r too low an e x t r a c t i v e pH i n the paper.

Tack Forces and

Ink F i l m S p l i t t i n g

O f f s e t l i t h o g r a p h i c p r i n t i n g involves an extra ink f i l m s p l i t between the p r i n t i n g p l a t e and the o f f s e t blanket. For t h i s reason and because the o f f s e t blanket conforms to the paper surface the ink f i l m t r a n s f e r r e d to paper can be thinner than i n letterpress printing. The pigment content of o f f s e t ink has to be greater than f o r l e t t e r p r e s s i n order to compensate f o r the thinner ink f i l m . Thicker ink f i l m s are a l s o not p o s s i b l e because h a l f t o n e dot d e f i n i t i o n would be l o s t due to ink spread on the f l a t l i t h o g r a p h i c p l a t e . O f f s e t inks a l s o contain more polymer r e s i n s (e.g. polyindene) than l e t t e r p r e s s inks i n order to improve ink t r a n s f e r and reduce waterlogging. A major d i s advantage of the higher pigment and polymer content of o f f s e t inks i s that much higher tack forces are generated during ink f i l m s p l i t t i n g than i n the l e t t e r p r e s s case. These tack f o r c e s are r e s p o n s i b l e f o r l i n t i n g of weakly bonded f i b r e s from the surface of newsprint and p i c k i n g of f i b r e s and coating from the surface of f i n e papers. Two important f a c t o r s i n the generation of tack forces i n a p r i n t i n g nip are c a v i t a t i o n and ink rheology.

Cavitation At low speed, f i l m s p l i t t i n g may be described by a simple hydrodynamic model. However, above a c r i t i c a l speed, the f i l m w i l l s p l i t by the formation and rupture of the ink f i l a m e n t s . The sub-atmospheric pressure developed i n the ink f i l m shown i n Figure 2 has never been d i r e c t l y measured or p r e c i s e l y located with respect to the nip centre. Nor i s i t even c e r t a i n whether the minimum occurs just before c a v i t a t i o n , or at the point of filament rupture.


20.

L Y N E AND ASPLER

413



C a v i t a t i o n w i l l commence when the cohesion of the ink i s overcome l o c a l l y , so that points of n u c l e a t i o n are h e l p f u l . M i l l e r and M y e r s — found by high speed photography of f i l m s s p l i t t i n g i n a low pressure chamber that while the f i l m s p l i t t i n g p a t t e r n was unchanged down to 0.1 atm. pressure, the work of s e p a r a t i o n increased due to the loss of the c o n t r i b u t i o n of a i r expansion w i t h i n the f i l m . In general, suspensions w i l l c a v i t a t e more r e a d i l y than t h e i r v e h i c l e f l u i d s , and large pigment p a r t i c l e s w i l l i n i t i a t e c a v i t a t i o n sooner, thus causing tack forces to be smaller. On the other hand, smaller p a r t i c l e s which cause c a v i t a t i o n l a t e r w i l l lead to f i n e r f i l a m e n t s , thereby improving p r i n t q u a l i t y by improving the uniformity of ink t r a n s f e r . The c a v i t i e s thus formed w i l l expand. This would suggest that the increase i n t o t a l surface energy due to the c r e a t i o n of new surface would be a c o n t r i b u t o r to the tack f o r c e . However, i n an a n a l y s i s of c a v i t y expansion w i t h i n a polyisobutene o i l , Hoffman and M y e r s — found that surface tension contributed only 7 percent and the v i s c o s i t y contributed 93 percent to the work of separation of the o i l f i l m . The f i n a l stage i n the f i l m s p l i t i s sequent breakage and r e c o i l of the ink e l a s t i c response of the ink, and surface bute to the force required to f r a c t u r e the

Ink Rheology and

the formation and subfilaments. Viscosity, tension forces c o n t r i filaments.

Tack Force

L e t t e r p r e s s news inks are pseudoplastic but not v i s c o e l a s t i c at the shear rates shown i n Figure 9. Polymer r e s i n s present i n o f f s e t ink formulations render them v i s c o e l a s t i c . This can be measured under o s c i l l a t o r y shear i n a rheometer such as the Mecha n i c a l Spectrometer: T(t) = G

f

Y

,

Q

s i n (a)t) + n Y

Q

cos

(wt)

(15)

where: T ( t ) = shear s t r e s s G'

= e l a s t i c storage

n'

• dynamic v i s c o s i t y

Y

= shear s t r a i n

Y

o

modulus

= shear rate

f

1

G i s a measure of the storage of e l a s t i c energy and n i s a measure of the viscous d i s s i p a t i o n of s t r a i n energy. These parameters are shown i n Figure 19 as a f u n c t i o n of the frequency of o s c i l l a t i o n u> of the cone i n the cone and p l a t e f i x t u r e of the Mechanical Spectrometer f o r an o f f s e t news ink. I t can be seen


REPROGRAPHIC

TECHNOLOGY


414

Figure 19. Rheological properties of a commercial lithographic offset news ink (viscosity -q, dynamic viscosity rf, and elastic storage modulus G') as measured in a platefixtureswith a cone angle of 2°20' (22).


20.

LYNE

A N D ASPLER


415

that the storage of e l a s t i c energy increases and the viscous d i s s i p a t i o n decreases as the frequency of o s c i l l a t i o n i s increased. Thus at the shear rates encountered i n p r i n t i n g , o f f s e t inks are l i k e l y to behave p r i n c i p a l l y as e l a s t i c m a t e r i a l s .


An adequate d e s c r i p t i o n of the rheology of sheet-fed o f f s e t i n k s — r e q u i r e s higher order (nonlinear) terms i n a d d i t i o n to those i n equation (15). Measurement of the normal forces may a l s o be u s e f u l i n e l u c i d a t i n g ink rheology. As mentioned p r e v i o u s l y , at very slow rates of f i l m s p l i t t i n g a simple hydrodynamic model can be used to describe the separation f o r c e . The v e l o c i t y of separation of the two surfaces m u l t i p l i e d by the v i s c o s i t y of the f l u i d gives a reasonable i n d i c a t i o n of the separation force i n t h i s case. However, i t should be borne i n mind that even pure polymeric l i q u i d s such as the polyisobutene used by the paper industry to measure the surface strength, or p i c k i n g r e s i s t a n c e , of paper i s non-Newtonian at high shear r a t e s . As shown i n Figure 20 i t i s d i f f i c u l t to decide the a c t u a l v i s c o s i t y of polyisobutene during p r i n t simulation tests. More importantly, the v e l o c i t y - v i s c o s i t y product does not p r e d i c t the separation force at the high rates of s e p a r a t i o n encountered i n p r i n t i n g due to c a v i t a t i o n e f f e c t s and complex flow behaviour i n the ink f i l a m e n t s . 2 5

E l a s t i c i t y at high shear rates i s a l s o bound to i n f l u e n c e tack f o r c e i n the n i p of an o f f s e t press. Besides shear rate the e l a s t i c i t y i s i n f l u e n c e d by the molecular weight and molecular s t r u c t u r e of the d i s s o l v e d polymers, and i n t e r a c t i o n s between the pigment p a r t i c l e s , the v e h i c l e , and the d i s s o l v e d polymer molecules. Of p a r t i c u l a r i n t e r e s t i s the e f f e c t of the molecular m o b i l i t y of s t i f f - c h a i n polymers such as polyindene on e l a s t i c i t y and tack force at the shear rates encountered i n an o f f s e t p r i n t ing n i p . Unfortunately, laboratory instruments such as the Inkometer and Tack-o-Scope do not measure tack f o r c e , but rather a complex combination of tack f o r c e , ink v i s c o s i t y , and the r h e o l o g i c a l properties of the e l a s t o m e r i c covering on t h e m e a s u r i n g roller^ I n v e s t i g a t i o n of the o r i g i n s or tack force i s a l s o hampered by a lack of knowledge surrounding the shear c o n d i t i o n s i n the p r i n t i n g nip, and measurements of e l a s t i c i t y and other r h e o l o g i c a l parameters may be misleading f o r the case of o f f s e t inks i f the shear rates do not correspond to those i n the p r i n t ing n i p . Kelha* et a l . ^ - ^1 developed a p a r a l l e l p l a t e tackmeter i n which the rate of a c c e l e r a t i o n of the separating p l a t e s was 50 m/s . While t h i s i s comparable to the rate of ink filament a c c e l e r a t i o n i n a commercial press the ink did not undergo shearing as i t would i n a p r i n t i n g n i p . However, they observed tack f o r c e s of about 5 mPa f o r l i n s e e d o i l - c a r b o n black o f f s e t inks having a f i l m thickness of 5 um. This tack force i s s l i g h t l y 2



416


oo o o o

300 IGT

L V oil 28 C

o 200

Shear Stress Pa

o

100

Silicon* oil AK 2000 25 C

o 0

A

A

A A

A

5000

10,000

Strain Rate s'

15,000 1

Figure 20. Shear stress developed in IGT polyisobutene pick test oil and a silicone oil as a function of shear rate in a Ferranti-Shirley cone and plate viscometer with a cone angle of 20'30" (22).


20.

LYNE AND ASPLER

417


lower than the specific bond strength between fibres in paper but could well cause fibre removal by bond peeling.


Summary Using a dynamic compression model for paper i t is possible to calculate the void fraction and effective capillary radius of paper under printing compression. A simple Poiseuille model can then be used to make an estimate for the hydraulic impression of ink into interfibre capillaries during the passage of paper through the printing nip. The role of surface chemistry in the wetting of paper and adhesion of ink during the passage of paper through the printing nip is not yet clearly understood. Calculation of the rate of capillary imbibition after the printing nip agrees well with the setting times for news inks and observations of opacity change in paper immediately after printing. Spreading appears to be the long-term mechanism by which ink vehicles migrate in paper. In the case of offset lithography the wetting of the surface of paper and subsequent sorption of fountain solution is important for the avoidance of printing problems such as ink refusal in multicolor printing, linting and picking, and interference with ink drying mechanisms when residual acid in paper is solubilized. Tack forces generated during ink film splitting are not well defined or easily measured with existing instrumentation. However, i t is suggested that ink tack is related to ink e l a s t i c i t y , or more fundamentally, to molecular mobility of s t i f f chain polymer additives to lithographic inks. Literature Cited 1.

L.M. Lyne and V. Madsen, Pulp Pap. Mag. Canada T523 (1964).

65,

2.

L . O . Larsson and P.O. Trollsas, in Paper in the Printing Process, W.H. Banks, ed., Pergamon, Oxford, (1965) p. 57.

3.

J - E . Levlin and L. Nordman, i b i d , p. 33.

4.

R. Chiodi and J . Silvy, in proceedings of Fourteenth EUCEPA Conference, Budapest, 1971; J . Silvy, C. Pannier and J . Veyre, in proceedings of Sixteenth EUCEPA Conference, Grenoble, 1976.

5.

R.W. Bassemir, American Ink Maker, 59 (2),

33 (1981).


(12),


418

REPROGRAPHIC TECHNOLOGY

6.

A. Surland, TAGA Proceedings, 1980, 222.

7.

W.C. Walker and J.M. Fetsko, American Ink Maker 33 (12), 38 (1955).

8.

P . J . Mangin, M.B. Lyne, D.H. Page and J . H . De Grâce, in Advances in Printing Science and Technology, V. 16, W.H. Banks, ed., in press.

9.

S.M. Chapman, Pulp Pap. Mag. Canada, 48 (Convention Issue), 140 (1947).

10.

J . Albrecht and M. Brune, in Recent Development in Graphic Arts Research, W.H. Banks, ed, Pergamon, Oxford (1971), p. 311.

11.

W.C. Bliesner, Tappi 53 (10), 1871, (1970).

12.

G. Blokhuis and A . E . Blogg, in Proceedings International Conference of Printing Res. Banks, ed. IPC Science & Tech., (1974), p. 53.

13.

M.B. Lyne, Tappi 59 (7), 102, (1976).

14.

G. Larocque, P.P. Mag. Can., 52 (Convention (1951).

15.

A . J . C . Yule, A . J . Howe, and J . H . Altman, Tappi 50 (7), 337, (1967).

16.

P. Oittinen, Advances in Printing Science Vol. 16, W.H. Banks, ed. in press.

17.

G.H. Dienes and H.F. Klemm, J . Appl, Phys. 17, 458 (1946).

18.

J . Colley and J . D . Peel, Paper Tech., 13 (10), 350 (1972).

19.

B. Hsu, Appl. S c i . Res. 10A, 277, (1961).

20.

Pira Visual Aid Kit, The Research Association for the Paper and Board Printing and Packaging Industries, Surry, U.K.

21.

J . H . De Grâce and P . J . Mangin, "A Mechanistic Approach to Ink Transfer through Simplified Models", Part I: "Model Subtrates" and Part II: "Model Inks", to be submitted to Tappi.

22.

J . S . Aspler, M.B. Lyne, J . M . Dealy and G.C. Pangalos, in Advances in Printing Science and Technology, V o l . 16, W.H. Banks ed., in press.

of the 12th Inst., W.H.

Issue),

166

and Technology,



20.

LYNE AND ASPLER


419

23.

P. Oittinen, D. Tech. Thesis, Technical University Helsinki; Acta Polytechnica Scandinavia No. 131 (1976).

24.

C O . Rosted, J . O i l C o l . Chem. Assoc. 55, 520 (1971).

25.

A . C . Zettlemoyer, R . F . Scarr and W.D. Schaeffer TAGA Proceedings, Part A, 1957, 75.

26.

L . O . Larsson 403.

27.

G.M. Dorris and D.G. Gray, J . 93, (1979).

28.

W.R. Haselton, Tappi, 37 (9) 404 (1954).

29.

P. Luner and M. Sandell, (1969).

30.

S. Lee and P. Luner, Tappi 55 (1), 116 (1972).

31.

M.B. Lyne, in Handbook of Physical and Mechanical Testing of Paper and Paperboard, R.E. Mark, ed; Marcel Dekker, Inc., in press.

32.

J . A . Bristow, Svensk Papperstiding,

33.

M.B. Lyne, Tech. Div.

34.

G. Suranyi, D.G. Gray, and D.A.I. Goring, Tappi 63 (4), 153 (1980).

35.

S. Takeyama and D.G. Gray, Cellulose Chem. and Technol., submitted for publication.

36.

R.W. Hoyland, in Fibre-Water Interactions in Paper-Making, Tech. Div. B.P.B.I.F., London (1978) p. 557.

37.

U. Lindqvist, S. Karttunen and J . Virtanen, in Advances in Printing Science and Technology, V o l . 16, W.H. Banks ed., in press.

38.

J . C . Miller (1958).

and R.R. Myers,

Trans.

Soc.

39.

R.D. Hoffman (1962).

and R.R. Myers,

Trans.

Soc. Rheol.

40.

L . Hellinckx and J . Mewis, Rheologica Acta 8, 519 (1969).

and G. Sunnerberg,

in Fibre B.P.B.I.F.,

TAGA Proceedings,

Coll.

J . Polym.

of

1972,

Interface S c i . 71 (1)

S c i . Part C. 28, 115

70 (19), 623 (1967).

Water Interactions in Paper-Making, London (1978), 641.

Rheol.


II,

77

IV, 197


420

REPROGRAPHIC TECHNOLOGY

41.

J . Mewls and F. Dobbels, Ind. Eng. Chem. Prod. Res. Dev., 20 (3), 515 (1981).

42.

V. Kelhä, M. Manninen, and P. Oittinen, J . Oil & Col. Chem. Assoc. 57 (6), 184 (1974).

43.

V. Kelhä, M. Manninen, and P. Oittinen, (1974).

44.

P. Oittinen, personal communication.

45.

J . Anhang, and D.G. Gray, J . Appl. Polym. S c i . , in press.

46.

M.B. Lyne and J . S . Aspler, "Wetting and the Sorption of Water by Paper Under Dynamic Conditions", to be presented at the 1982 TAPPI Paper Physics Seminar, Pointe Claire, submitted to Tappi.

47.

W.A. Zisman, in Contact Angle Wettability and Adhesion, ACS Advances in Chemistry Series no. 43, (164) p. 1.

48.

M.B. Lyne and A. Parush, "The Print Quality of Offset and Letterpress Newsprint" given at the 1981 ANPA-CPPA conference on Newsprint in the Pressroom, submitted to Pulp and Paper Mag. of Can.

49.

J . F . Oliver and S.G. Mason, in Fundamental Properties of Paper Related to Its Uses, Tech. Div. B . P . B . I . F . , London (1976) p. 428.

50.

R.W. Hoyland, P. Howarth and R. F i e l d , in Fundamental Properties Paper Related to Its Uses, Tech. D i v . B . P . B . I . F . , London (1976) p. 464.

Tappi 57 (4), 86

RECEIVED April 30, 1982


Ink-Paper Interactions in Printing: A Review - ACS Publications

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